Thermochemical regeneration with soot formation

ABSTRACT

Operation of a thermochemical regenerator to generate soot or to increase the amount of soot generated improves the performance of a furnace with which the thermochemical regenerator is operated.

RELATED APPLICATION

This application claims the benefit of U.S. Non-Provisional applicationSer. No. 15/183,879, filed on Jun. 16, 2016, which claims the benefit ofU.S. Provisional Application Ser. No. 62/181,528, filed on Jun. 18,2015, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to combustion in furnaces such asglassmelting furnaces wherein material is fed into the furnace and isheated and/or melted by the heat of combustion that occurs within thefurnace.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,113,874 discloses heat recovery methods useful withfurnaces employing regenerators wherein a stream of combustion productsformed in the furnace is passed through a first regenerator to heat thefirst regenerator and cool the combustion products, and then a portionof the cooled combustion products is combined with fuel to form amixture which is passed through a second heated regenerator and where itundergoes an endothermic reaction to form syngas that then passes intothe furnace and is combusted.

The present invention is an improvement in the methods disclosed in thatpatent, whereby it has unexpectedly been found that the efficient heatrecovery afforded by these methods can be improved and other benefitsdescribed herein can be realized. In particular, the present inventionencourages the formation of significant amounts of soot at certainstages, whereas the aforementioned U.S. Pat. No. 6,113,874 teaches thatsoot is something to be minimized and, if produced, to be removed.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of carrying outcombustion in a furnace, comprising

(A) combusting fuel in a furnace to produce gaseous combustion products,and (B) alternately (1) (i) passing gaseous combustion products from thefurnace into and through a cooled first regenerator to heat the firstregenerator and cool said gaseous combustion products,

(ii) passing at least a portion of said cooled portion of gaseouscombustion products from said first regenerator, and hydrocarbon fuel,into a heated second regenerator,

(iii) reacting the gaseous combustion products and the fuel in thesecond regenerator in an endothermic reaction under conditions effectiveto form syngas comprising hydrogen and carbon monoxide and to form sootwhich is entrained in the syngas, and thereby cooling the secondregenerator; and

(iv) passing said syngas and entrained soot from said second regeneratorinto said furnace and combusting the syngas in the furnace with one ormore oxidant streams injected into said furnace; and

(2) (i) passing a portion of the gaseous combustion products from thefurnace into and through a cooled second regenerator to heat the secondregenerator and cool said portion of the gaseous combustion products,

(ii) passing at least a portion of said cooled portion of gaseouscombustion products from said second regenerator, and hydrocarbon fuel,into a heated first regenerator,

(iii) reacting the gaseous combustion products and the fuel in the firstregenerator in an endothermic reaction under conditions effective toform syngas comprising hydrogen and carbon monoxide and to form sootwhich is entrained in the syngas, and thereby cooling the firstregenerator, and

(iv) passing said syngas and entrained soot from said first regeneratorinto said furnace and combusting the syngas in the furnace with one ormore oxidant streams injected into said furnace.

Another aspect of the present invention is a method of carrying outcombustion in a furnace, comprising

(A) combusting fuel in a furnace to produce gaseous combustion products,

(B) passing at least a portion of the gaseous combustion products fromthe furnace, and hydrocarbon fuel, into a duct,

(C) reacting the gaseous combustion products and the fuel in the duct inan endothermic reaction under conditions effective to form syngascomprising hydrogen and carbon monoxide and to form soot which isentrained in the syngas, and

(D) passing said syngas and entrained soot from said duct into saidfurnace and combusting the syngas in the furnace with one or moreoxidant streams injected into said furnace.

As used herein, “soot” is carbon-containing particulate matter. Soot mayexhibit a wide range of morphological characteristics including size,shape, surface structure, and chemical compositions. For example, sootparticles can be porous and contain 0.5% to 2% of hydrogen with themajority of the rest as carbonaceous materials. The size of a sootparticulate can range from 50 to 650 angstroms (Å); however, if sootparticles agglomerate, then the final soot masses can be in filamentform and reach as long as 3000 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are schematic representations of different aspects of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described herein in particular detail with respect to apreferred type of furnace, namely one that employs a heat recoveryprocess which recaptures usable heat from high temperature flue gasexhaust streams. This heat recovery process proceeds in two cycles,which are referred to herein as the flue cycle and the reforming cycle.These two cycles are performed alternatingly in two or morechecker-filled regenerators. The heat recovery process is preferablycarried out in association with furnaces and other combustion deviceswhich employ “oxy-fuel” combustion processes, i.e. combustion of fuelwith gaseous oxidant comprising an oxygen content of at least 50 vol. %oxygen, and preferably at least 80 vol. % oxygen, more preferably atleast 90 vol. % oxygen, and even at least 99 vol. % oxygen, because theflue gases produced by oxy-fuel combustion have higher H₂O and CO₂concentrations, both of which promote the endothermic reformingreactions that are utilized in the method of this invention. During theflue cycle, the checkers in a first regenerator extract and store heatfrom a high temperature flue gas which is fed from the furnace into andthrough this regenerator. Then, in the reforming cycle, from the cooledflue gas that exits the first regenerator, a portion (which is referredto herein as Recycled Flue Gas or RFG) is fed into another (second)regenerator and mixed with a stream of fuel (referred to herein asReforming Fuel or RF). In the following description, pure methane (CH₄)is described as reforming fuel for purposes of illustration. Othersatisfactory fuels include any combustible gas, gas mixture, orvaporized liquid fuels including, but not limited to, natural gas,propane, and LPG (liquefied petroleum gas).

In the reforming cycle, the RFG/Reforming Fuel mixture enters the secondregenerator in which the checker has already been heated, as describedherein, and flows through it towards the furnace. The temperature of theRFG/RF mixture passing through the second regenerator continues toincrease by extracting heat from the already pre-heated checker. As theRFG/RF mixture passes through the second regenerator, it reaches atemperature at which reforming reactions begin to occur and continue tooccur, producing products including H₂ and CO. The reforming reactionsare endothermic and the heat needed to promote the reforming reactionsis absorbed from the heated checker. The reforming reactions produce amixture of soot entrained in a gaseous composition, which typicallycomprises one or more components such as such as H₂, CO, and unreactedgases comprising H₂O, CO₂ and CH₄. The gaseous composition thus producedmay also be called “syngas” herein. The mixture of gaseous products andsoot emerges from the second regenerator into the furnace wherein thecombustible gaseous components are combusted with oxidant to providethermal energy for heating and/or melting material in the furnace. Thiscombustion may combust and gasify a portion of the soot inside a flameboundary, but eventually all of the soot in the syngas is eitheroxidized directly by excess O₂ in furnace flue gas to CO₂ and H₂O, orgasified to CO and H₂ firstly by H₂O and CO₂ in the furnace flue. CO andH₂ gases produced by the soot gasification are further oxidized to CO₂and H₂O by excess O₂ in the furnace. Therefore, the gaseous combustionproducts (flue gas) produced in the furnace and discharged from thefurnace such as in a duct for additional heat recovery contain littleor, preferably, no soot. This combustion process, where most or all ofthe soot in the syngas is continuously burned and gasified within aflame boundary, produces a luminous flame having superior heat transfercharacteristics and benefits, as described more fully below.

For a typical cruciform checker with a square gas passage of 15 cm by 15cm size, the syngas mixture produced by the endothermic reactionspreferably contains 0.1 to 20 grams of soot per cubic meter of themixture (g/m³), more preferably at least 1.0 g/m³ of soot, and yet morepreferably 1.0 to 10 g/m³ of soot. Larger checker gas passages requireless soot and conversely smaller gas passages require more soot for thesame heat transfer benefit.

To promote formation of the desired amounts of soot in the mixture thatis produced by the endothermic reforming reactions, the syngastemperature exiting from the regenerator in which the endothermicreactions occur, should preferably be at a temperature of at least 1000F, and the residence time therein should be at least 0.1 seconds, up to10 seconds. Formation of the desired amounts of soot is also promoted byoperating with a low ratio of recycled flue gas to natural gas asreforming fuel, preferably at a ratio (by volume at 60 F) less than 2:1,more preferably less than 1:1, or even less than 0.5:1. Reducing theexcess O₂ content of the furnace combustion gases will correspondinglyreduce the O₂ concentration in the RFG, thus promoting soot formationduring the endothermic reforming reactions.

After a length of time, the operation of the two regenerators isreversed, i.e., the regenerator that was used in the flue cycle isswitched to the reforming cycle, and the regenerator that was used inthe reforming cycle is switched to the flue cycle. After a furtherperiod of time, the operation of the two regenerators is reversed again.The timing of the reversals can be determined by elapsed time, or byother criteria such as the temperature of the flue gas exiting from thefirst regenerator that is in flue cycle. The reversal process is carriedout according to a predetermined mechanism and plan, wherein valves aresequenced to open and close based on specific timings.

The operation and control of the present invention is described below inconjunction with FIGS. 1 to 3. An end-port fired glass furnace (10)fitted with two regenerators in end wall (3) is used as an example.

As shown in FIG. 1, end-port glass furnace (10) has a feed station (20)where feed material (30) comprising solid glassmaking materials (knownas batch and/or cullet) are charged into the furnace to be heated andmelted. The flow of molten glass out of furnace (10) is represented as(90). The furnace (10) is equipped with first regenerator (100) on thefurnace left side and second regenerator (200) on the furnace rightside. Vertical cross-sectional views of the two regenerators aredisplayed in more detail in FIGS. 2 and 3.

As seen in FIG. 2, regenerator (200) is in the flue cycle wherein fluegas stream (50) from the interior of furnace (10) enters port neck (240)and then flows to the top space (530) of regenerator (200) past anoxygen analyzer (250). The flue gas stream heats checkers (representedas (520)) as it flows through passages between the checkers withinregenerator (200), and enters chamber bottom space (500) through gaspassages (515) supported on arch (510) which also supports the weight ofthe whole bed of checkers. As seen in FIG. 1, a portion (52) of the fluegases produced in furnace (10) may be by-passed to conduit (70) througha partially opened valve (350) then enters stack (340) to exhaust, bywhich is meant that it does not re-enter the furnace but instead isdischarged to the atmosphere and/or conveyed to one or more otherstations for storage and/or further treatment or any combination of suchdestinations. For maximum heat recovery, it is preferred that valve(350) is closed so that essentially all the furnace flue gas goes toregenerator (200) as flue gas stream (50).

As seen in FIGS. 1 and 2, the cooled flue gas stream (201) exits theregenerator (200) in conduit (260), passes through an open valve (210)and oxygen sensor (310), and then enters the suction-side of blower(300). The majority of the flue gas (301) leaving the pressure-side ofthe blower passes through a damper (330) then a flow meter (332), andfinally is directed into stack (340) through which this flue gas leavesthe system to exhaust as defined herein. A portion (303) of the flue gasis recycled to the bottom of regenerator (100) by passing throughconduit (320) and valve (360). This is Recycled Flue Gas (RFG). Its flowis metered by a flow meter (322). Reforming fuel which is to be fed tothe second regenerator (100) is supplied by a conduit (130) throughvalve (120).

Suitable reforming fuels include methane (which is preferred) as well asany other combustible gas, gas mixture, or vaporized liquid fuelsincluding, but not limited to, natural gas, propane, and LPG (liquefiedpetroleum gas).

As seen in FIG. 3, the reforming fuel (RF) from stream (130) intersectsand mixes with the RFG (303) at location (127) in conduit (128) whichalso communicates with the bottom space (400) of regenerator (100). ThisRFG/RF mixture enters the already pre-heated checker pack (420) ofregenerator (100) through gas passages (415) on arch (410). Regenerator(100) has already been heated in a previous cycle by passage of flue gasfrom the furnace into and through the regenerator (100). The temperatureof the RFG/RF mixture increases as it flows through the checker pack ofregenerator (100). When the temperature of the RFG/RF reaches reformingtemperature, endothermic reforming reactions occur in which thereforming fuel (e.g. CH₄) reacts with CO₂ and H₂O in the RFG and formsCO, H₂, and soot. The required heat for the endothermic reforming andsoot-forming reactions is taken from the heated checkers. The reformingreaction continues as the RFG/RF mixture continues to travel toward thetop space (430). The gaseous, soot-containing stream (425) (referred toherein as a “reformed” or “syngas” gas stream) exits from the top ofchecker pack (420). Stream (425) has high temperature and may includespecies such as CO, H₂, soot, unreacted CH₄, and unreacted CO₂ and H₂O.The stream (425) passes through port neck (140) and oxygen sensor (150),and enters furnace (10). This stream exits checker pack (420) attemperatures for example ranging from 1800 F to 2500 F.

Oxidant for combustion of the syngas is supplied by a conduit (135) withan opened valve (115). This oxidant can be air, or it can have an oxygencontent higher than that of air, i.e. at least 21 vol. %, and preferablyequal to or higher than 80 vol. %, more preferably equal to or higherthan 90 vol. %, or even at least 99 vol. %.

Typically, the heat recovery process proceeds with one regenerator inthe flue cycle and one regenerator in the reforming cycle, as seen inFIG. 1, for about 20 to 40 minutes or until the checkers in thereforming regenerator are too cold to provide sufficient heat to promotethe desired endothermic chemical reactions. At that point, and nowcontinuing with the description herein where regenerator (200) was inthe flue cycle and regenerator (100) was in the reforming cycle, furnace(10) undergoes reversal in which regenerator (200) is transitioned tothe reforming cycle for heat recovery and regenerator (100) istransitioned into the flue cycle for heat accumulation. Before thereversal, remaining syngas in regenerator (100) is to be purged tofurnace (10). In this instance, reforming fuel supplied to theregenerator is terminated at first by closing valve (120) while lettingthe flow of RFG from blower (300) continue. Remaining syngas inregenerator (100) is purged by the RFG for a specified amount of time sothat nearly all the syngas in the regenerator is expelled to the furnaceand combusted to completion.

Upon reversal, the flue gas from the furnace passes through regenerator(100), and a portion thereof passes to exhaust (as defined herein) whilea portion or the balance is mixed with fuel and the mixture is passedthrough regenerator (200) and into the furnace. Valve (110) which hadbeen closed is opened, valve (210) is closed, and valve (360) is closedand valve (380) is opened, to permit heated flue gas to pass fromregenerator (100) toward and through blower (300), and to permit aportion (303) of this flue gas to pass into regenerator (200) after itis mixed with reforming fuel (230) which enters through valve (220)which had been closed but now is opened. Valve (115) which had been openis closed, as no combustion aided by oxidant through valve (115) occursin this phase, and valve (225) is opened. The resulting mixture ofreforming fuel and recycled flue gas undergoes in regenerator (200) theendothermic reforming and soot-forming reactions which had occurred inregenerator (100) in the previous cycle as described herein, to producestream (425) of syngas and soot which passes into furnace (10) where itis combusted with oxidant (235) that is fed through valve (225).

During the heat recovery process, furnace (10) may be co-fired withother burners such as (60) and (65) such that both syngas flame (40) andburner flames (62) and (64) co-exist. In addition, burners (60) and (65)may or may not be firing during the reversal process when the reformingregenerator (i.e. (100) or (200) as the case may be) is undergoing thepurging sequence described above. For maximum heat recovery, it ispreferred that burners (60) and (65) are not co-firing with the syngasflame (40). It is also preferred that during the purging sequence,burners (60) and (65) are not firing.

The soot content of the gas-soot stream (425) can be further enhanced byadding soot and/or by adding additional carbonaceous soot precursormaterial into the unit in which the endothermic reforming andsoot-forming reactions occur. This is illustrated in FIG. 1, wherestream (315) represents an external source of soot, which may be astream of soot entrained in air or other suitable carrier gas, or may bea stream comprising hydrocarbons which have a high tendency to crackunder the conditions of the endothermic reaction, forming additionalsoot. Examples of such hydrocarbons include polycyclic aromatichydrocarbons (PAH) such as naphthalene, aromatic hydrocarbons such asbenzene and toluene, and aliphatic hydrocarbons such as acetylene andethylene. Stream (315) is fed to connect with conduit (320) such thatstream (315) mixes with RFG when valve (316) is opened. The resultingmixed stream is then mixed with reforming fuel alternately from conduits(130) and (230), to form a mixture that is alternately fed into theheated regenerator (100) and (200) respectively wherein the reformingand soot-forming reactions occur as described herein.

A preferred way to inject additional soot is to inject it into thehigher temperature region of the reforming regenerator (top of checkerpack (420) in FIG. 3). In this region, the reforming reactions arefaster due to the higher syngas temperature. If extra heat is extractedfrom the heated checkers due to enhanced checker to syngas radiativeheat transfer by soot injection, this extra heat could promoteendothermic reforming reactions to high degrees of completion such thatmore heat can be recovered by a fixed volume of checkers. One difficultyof doing this is soot has to be distributed into the syngas ratheruniformly. The checker pack (420) may have to be divided and structuredinto at least two sections with a cavity in between. Soot would beinjected into the cavity and mixed well with the syngas coming out ofthe lower checker section. This syngas/external soot mixture then flowsinto the second checker section and upward for further reforming withthe help of the injected soot for enhanced heat transfer.

Flue gas stream (50) may contain components of glass making materials inparticulate form such as limestone and dolomite (CaCO₃, MgCO₃) andvolatilized alkalis such as sodium and potassium, in addition to gaseousspecies such as SO₂, CO₂, O₂, N₂, and H₂O. Fine raw materials inbatch/cullet (30) can be entrained into the flue gas stream and carriedover to the regenerator (200) top. Fine particulates are also formed dueto decrepitation of dolomite and limestone particles upon heating.Particles of condensed matter such as alkali sulfates (Na₂S_(O) 4,K₂SO₄) can form typically from 1150 C to 800 C gas temperature when theflue gas is cooled down in regenerator (200) which is in the flue cycle.Some of this carried-over and condensed particulate matter may depositon regenerator walls and checkers and may be present in the cooled fluegas stream (201). Field measurement of particulate matter in flue gasstreams of oxyfuel fired glass furnaces had showed concentrationsranging from 0.2 g to 1.0 g per Nm³ (normal cubic meter defined at zero° C.) of flue gas volume. The RFG stream (303) advancing towardsregenerator (100) for reforming reactions also contains about the samelevel of particulate matters as in stream (201).

This particulate matter in RFG stream (303) will contribute to theenhancement of radiative heat transfer also between the already heatedchecker and the colder RFG/RF mixture in regenerator (100) duringreforming. The above beneficial heat transfer effect is especiallyadvantageous in the lower portion of checker pack (420) in regenerator(100) where the temperature of the RFG/RF mixture is lower than 1000 Fand the reforming reactions are slower in speed thus less soot isproduced by the reforming reactions. Thus it is preferred that theparticulate matter in RFG stream is not filtered prior to passing intocooled regenerator (200).

The present invention provides numerous benefits.

It has been determined that, quite unexpectedly, this method improvesthe overall heat transfer rate to the reactants in the endothermicreforming and soot-forming reactions in the regenerator or other duct inwhich those reactions occur, even though the concentrations of gaseousspecies (such as CH₄, CO₂, and H₂O) that would be expected toparticipate in the thermal radiation heat transfer within theregenerator are decreasing as the endothermic reforming proceeds. As aresult of the method of the present invention, the reforming reactionproceeds further and converts more of the reactants at any given set ofinitial conditions (i.e. temperature within the regenerator at the pointwhen flow of the RFG/RF mixture into the regenerator begins).

In addition, the present invention provides improved flame and heattransfer characteristics within the furnace upon combustion of themixture stream (of syngas and soot) in the furnace. When the soot-ladensyngas is combusted in the furnace and forms a flame, the emissive powerof the flame (i.e., total radiation energy emitted per unit surface areaof flame) is increased due to the existence of the soot particles insidethe flame envelope (relative to a flame upon combustion of a streamcontaining no soot, or containing lesser amounts of soot than what thepresent invention provides). This increased emissive power results inincreased heat transfer to the material that is in the furnace to beheated and/or melted, by virtue of the increased radiative heattransfer.

This feature is especially advantageous in embodiments wherein theheating and/or melting of the solid material in the furnace can causefine particles of material to be entrained into the gaseous combustionproducts that are formed by the combustion in the furnace. The presenceof such particles in the gaseous combustion products is undesirable, asthey can form deposits on surfaces of the interior of the furnace or oninner surfaces of ducts connecting to the furnace, and they can bepresent in the flue gas that is exhausted to the atmosphere or to otherlocations within the overall operation. Entrainment of fine particles ofmaterial from the solid material being heated and/or melted in thefurnace can occur when the material itself contains fine particulatematter, or when particles of the material decrepitate on being heated toproduce fine particles into the gaseous atmosphere over the solidproduct. Such results can occur, for example, when the solid materialbeing heated and/or melted in the furnace is batch and/or culletmaterial that is the typical feed material in the case of a glassmeltingfurnace. The increased emissive power of the flame that is produced bythe present invention reduces this problem in that it melts (“glazes”)surfaces of the material in the furnace, and does so more quickly thanwould otherwise be the case. This greatly reduces the tendency ofparticulate material that is present to be entrained into the gaseouscombustion products above the materials, and thereby reduces theavailability of entrained particulate material to collect in regeneratorpassages (which would lead to plugging of the passages) or to escapeinto the atmosphere (which would cause environmental emission problems).

EXAMPLE

The effectiveness of the present invention to provide in-situ sootgeneration to accelerate regenerator checker to gas radiative heattransfer during the thermal-chemical heat recovery process, isillustrated in this example and in the accompanying table below (whichare provided for purposes of illustration and not limitation).

Five types of flue gas compositions were selected to illustrate theeffect of radiative species (CO₂, H₂O, CO, CH₄, and soot) on regeneratorchecker to gas radiation heat transfers. For Cases 1 through 5, the sameassumptions were made in each case as indicated in the following: gastemperatures at 1120 C, checker temperatures at 1150 C, radiationmean-beam lengths in checkers at 0.14 m, checker surface emissivititesat 0.4, and the total pressure of the concerned heat transfer processwas at 1 atm.

In Case 1, flue gas from an air-fired combustor which had 9.1% CO₂ and18.2% H₂O was selected as a reference case. Using available informationshown in the column for Case 1 and an emissivity model developed forluminous flames (not shown), gas emissivity was calculated to be 0.092.Using another 1(sink)-1(grey gas)-1(black surface) radiative heatexchange model (not shown) as an approximation, net radiative heat fluxfrom the checker to the flue gas was calculated as 699 w/m².

In Case 2, flue gas was from an oxy-fired combustor therefore it hadhigher CO₂ (32.9%) and H₂O (65.8%) concentrations. Calculated gasemissivity and net radiative heat flux to the checker surface was 0.187and 1421 w/m², respectively. As seen in Table 1, the net radiative heatflux received by the oxy flue (Case 2) is 2.03 times of that of the airflue (Case 1). The higher net heat flux of Case 2 can be attributed tothe higher concentrations of CO₂ and H₂O in the flue.

Cases 1 and 2 are shown here for reference, to illustrate the superiorunexpected results obtained with the present invention as exemplified inCases 3, 4 and 5.

Cases 3, 4 and 5 simulate checker to gas radiative heat transfers underan assumed reforming condition during thermal chemical reforming. Syngascompositions in checker passages were assumed to have 6% CO₂, 30% H₂O,2% CH₄, 40% Hz, and 22% CO for all the three cases. In Case 3, it waspresumed that the syngas contained no soot. Calculated syngas emissivitywas 0.143 and a net heat flux of 1086 w/m² was obtained, which wasunexpectedly lower than 1421 w/m² of the oxy flue heat transfer of Case2. The lower net heat flux of Case 3 when compared to that of Case 2 wasdue to lower concentrations of participating radiative species (total ofCO₂, CO, CH₄, and H₂O). As described in previous sections of thisdisclosure of the invention, the lower gas emissivity of Case 3 is theresult of thermal chemical reforming which depleted CO₂ and H₂O gasesand generated a large amount of H₂ gas and the hydrogen gas does notparticipate in nor contribute to the checker-to-gas radiation heattransfer.

To enhance heat transfer during thermal chemical heat recovery process,the reforming cycle was operated under preferred conditions such that aportion of the reforming fuel generated soot. As listed in Table 1,syngas compositions for Cases 4 and 5 were kept the same as those ofCase 3, except that different amount of soot concentrations wereintroduced. For explaining purpose, soot concentrations for the twocases were set at 1 g/m³ for Case 4 and 2 g/m³ for Case 5. In Case 4,the calculated total emissivity for the gas/soot mixture was 0.345 whichis significantly higher than that of the syngas without soot (Case 3)and the oxy-flue example (Case 2). This higher emissivity contributed tothe higher heat flux (2621 w/m²) received by the syngas. As seen in Case5 when 2 g/m³ of soot were formed during reforming, emissivity of thegas/soot mixture increased to 0.489. The corresponding net heat fluxreceived by the syngas was 3715 w/m² which is 5.31 times higher thanthat of the reference case (Case 1).

In summary, this example has demonstrated clearly the effectiveness ofgenerating in-situ soot during the thermochemical heat recovery process,for the purpose of enhancing overall checker-to-gas heat transfer rateswhich in turn improves waste heat recovery efficiency.

Case 1 Case 2 Case 4 Case 5 Air- Oxy- Case 3 Syngas Syngas fired firedSyngas with less with more flue gas flue gas without soot soot soot Gascomposition: CO₂ (%) 9.1 32.9 6 6 6 H₂O (%) 18.2 65.8 30 30 30 O₂ (%) 11.3 0 0 0 CH₄ (%) 0 0 2 2 2 H₂ (%) 0 0 40 40 40 CO (%) 0 0 22 22 22 Soot(g/m³) 0 0 0 1 2 Gas temperature (C.) 1120 1120 1120 1120 1120 Gaspressure (atm) 1 1 1 1 1 Mean beam length 0.14 0.14 0.14 0.14 0.14 (m)Gas or Mixture 0.092 0.187 0.143 0.345 0.489 emissivity Checker 11501150 1150 1150 1150 temperature, (C.) Checker emissivity 0.4 0.4 0.4 0.40.4 Net heat flux by 699 1421 1086 2621 3715 thermal radiation (w/m²)Ratio of net heat 1 2.03 1.55 3.75 5.31 flux

What is claimed is:
 1. A method of carrying out combustion in a furnace,comprising (A) combusting fuel in a furnace to produce gaseouscombustion products, and (B) alternately (1) (i) passing gaseouscombustion products from the furnace into and through a cooled firstregenerator to heat the first regenerator and cool said gaseouscombustion products, (ii) passing at least a portion of said cooledportion of gaseous combustion products from said first regenerator, andhydrocarbon fuel, into a heated second regenerator, (iii) reacting thegaseous combustion products and the fuel in the second regenerator in anendothermic reaction under conditions effective to form syngascomprising hydrogen and carbon monoxide and to form soot which isentrained in the syngas, and thereby cooling the second regenerator; and(iv) passing said syngas and entrained soot from said second regeneratorinto said furnace and combusting the syngas and entrained soot in thefurnace with one or more oxidant streams injected into said furnace; and(2) (i) passing a portion of the gaseous combustion products from thefurnace into and through a cooled second regenerator to heat the secondregenerator and cool said portion of the gaseous combustion products,(ii) passing at least a portion of said cooled portion of gaseouscombustion products from said second regenerator, and hydrocarbon fuel,into a heated first regenerator, (iii) reacting the gaseous combustionproducts and the fuel in the first regenerator in an endothermicreaction under conditions effective to form syngas comprising hydrogenand carbon monoxide and to form soot which is entrained in the syngas,and thereby cooling the first regenerator, and (iv) passing said syngasand entrained soot from said first regenerator into said furnace andcombusting the syngas and entrained soot in the furnace with one or moreoxidant streams injected into said furnace.
 2. A method of carrying outcombustion in a furnace, comprising (A) combusting fuel in a furnace toproduce gaseous combustion products, (B) passing at least a portion ofthe gaseous combustion products from the furnace, and hydrocarbon fuel,into a duct, (C) reacting the gaseous combustion products and the fuelin the duct in an endothermic reaction under conditions effective toform syngas comprising hydrogen and carbon monoxide and to form sootwhich is entrained in the syngas, and (D) passing said syngas andentrained soot from said duct into said furnace and combusting thesyngas and entrained soot in the furnace with one or more oxidantstreams injected into said furnace.
 3. The method of claim 1 wherein thefurnace contains material that contains fine particulate matter ormaterial that upon being heated in the furnace produces fine particulatematter by decrepitation, and the combustion of the syngas and entrainedsoot in the furnace accelerates the formation of a glassy layer on saidmaterial which thereby reduces the fine particulate matter from beingentrained into gaseous combustion products in the furnace.
 4. The methodof claim 1 further comprising adding to (a) the cooled portion ofgaseous combustion products, and hydrocarbon fuel, which is passed intothe heated second regenerator, or to (b) the cooled portion of gaseouscombustion products, and hydrocarbon fuel, which is passed into theheated first regenerator, or to both (a) and (b), additional materialwhich forms soot in said heated regenerator.
 5. The method of claim 1further comprising adding soot to (a) the cooled portion of gaseouscombustion products, and hydrocarbon fuel, which is passed into theheated second regenerator, or to (b) the cooled portion of gaseouscombustion products, and hydrocarbon fuel, which is passed into theheated first regenerator, or to both (a) and (b).
 6. The method of claim1 wherein said hydrocarbon fuel is natural gas and the molar ratio ofthe portion of said cooled portion of gaseous combustion products fromsaid first regenerator passed into a heated second regenerator and saidhydrocarbon fuel is less than 0.5:1.
 7. The method of claim 1 whereinsaid cooled portion of gaseous combustion products from said firstregenerator passed into a heated second regenerator contains more than0.1 g per Nm³ of particulates and enhances radiative heat transfer insaid second regenerator.
 8. The method of claim 1 wherein the gaseouscombustion products passed from the furnace into the cooled firstregenerator and the gaseous combustion products passed from the furnaceinto the cooled second regenerator contain no soot.
 9. The method ofclaim 2 wherein the furnace contains material that contains fineparticulate matter or material that upon being heated in the furnaceproduces fine particulate matter by decrepitation, and the combustion ofthe syngas and entrained soot in the furnace accelerates the formationof a glassy layer on said material which thereby reduces the fineparticulate matter from being entrained into gaseous combustion productsin the furnace.
 10. The method of claim 2 further comprising adding tothe cooled portion of gaseous combustion products, and hydrocarbon fuel,which is passed into the duct, additional material which will form sootin said duct.
 11. The method of claim 2 further comprising adding sootto the cooled portion of gaseous combustion products, and hydrocarbonfuel, which is passed into the duct.
 12. The method of claim 2 whereinthe gaseous combustion products passed from the furnace into the ductcontain no soot.